(1) Field of the Invention
The present invention relates to a flash fixation process using a toner for visualizing an electrostatic latent image in electrophotography or the like.
(2) Description of the Related Art
In general, in the electrophotography processes used in copying machines or laser printers, an electrostatic charge is uniformly applied to a photoconductive insulator layer, the insulator layer is exposed imagewise to light to partially remove the electrostatic charge and form an electrostatic latent image thereon, a fine powder called "toner" is caused to adhere to the electrostatic charge-remaining area to visualize the latent image and form a toner image (this operation is called "development"), and the toner image is fixed to a recording paper to obtain a print (this operation is called "fixation").
The toner is a finely divided powder formed by dispersing a colorant and a charge control agent in a binder resin composed of a natural or synthetic polymeric substance and pulverizing the dispersion to a size of about 1 to about 30 .mu.m. Ordinarily, this finely divided powder is mixed with a carrier such as iron powder or glass beads to form a developer, and such developer is used for the development. However, the above-mentioned toner image is composed solely of the toner.
The fixation is an operation of melting the toner of the toner image and fixing it to a recording paper. The known fixation methods include, heat-pressure fixation, oven fixation, pressure fixation, solvent fixation and light fixation. In flash fixation which is a typical example of light fixation processes, fixation is effected by a flash of a discharge tube such as a xenon flash lamp, and this flash fixation process is preferred because of the following characteristics.
(1) Since the fixation is a non-contact fixation, resolution in the developed image is not degraded by the fixation.
(2) There is no wait time after application of electric power, and quick starting is possible.
(3) Even if jamming of recording paper occurs in the fixing zone by system-down, ignition is not caused.
(4) Fixing is possible irrespective of the material and thickness of recording papers, and therefore, fixation can be made on pasted papers, preprinted papers, papers differing in thickness and the like.
However, only black toners have been used in flash fixation processes until the present. Since coloration is now advanced in office automation (OA) equipment, it is eagerly desired to provide color toners applicable for flash fixation processes as soon as possible.
The conventional process for fixing a toner 1 to a recording paper 2 by the flash fixation will now be described with reference to FIGS. 1a and 1b.
As pointed out hereinbefore, when a toner image is transferred onto the recording paper 2, the toner 1 adheres in the powdery state to the recording paper 2 to form an image as shown in FIG. 1a, but this image may be crumbled, for example, by rubbing with the fingers. If a flash 3 of a discharge tube such as a xenon flash lamp is applied to the toner image, the toner 1 absorbs the energy of the flash 3 to convert it to a thermal energy, and hence, the temperature is elevated and the toner is softened and melted and adheres to the recording paper 2. After the extinction of the flash 3, the temperature falls and a fixed image 4 is formed by solidification, as shown in FIG. 1b, whereby the fixation is completed. Now the fixed image 4 on the recording paper 2 may no longer be crumbled, for example, by rubbing with the fingers.
It is important that the toner 1 should be melted and adhered closely to the recording paper 2. Accordingly, it is necessary for the toner 1 to absorb a sufficient amount of light energy, including the thermal energy which will diffuse to the outside and make no contribution to the elevation of the temperature of the toner, from the flash 3.
As shown in FIG. 2, the spectral distribution of the flash from a xenon flash lamp ordinarily used as the discharge tube for flash fixation processes covers a broad area ranging from the ultraviolet region to the infrared region, but the spectral intensity of the flash is strong only in the near infrared region of 800 to 1000 nm and the spectral intensity of the flash is relatively weak in other regions including in the visible region of 400 to 800 nm. Accordingly, to provide a good fixing property, it is preferable that the toner should have a good absorbing property to rays of the near infrared region.
The polymeric substances typically used as the binder, which is the main component of the toner 1, generally have very low absorption properties with respect to light energy of the visible and near infrared regions, and colorants other than black colorants have acceptable absorption characteristics with respect to rays of the visible region but have poor absorption characteristics to rays of the near infrared region. Accordingly, a toner comprising such a binder resin and a non-black colorant in combination is, for all practical purpose, not capable of being melted by irradiation with the flash 3. For this reason, useful color toners for flash fixation processes have not been available in the past.